Safe Speed has discovered
a critical flaw in the current rules for speed camera placement

Introduction

Most of the UK now participates in Camera
partnerships. In this scheme (sometimes called "hypothecation"
or "netting off") fine cash from cameras may be recovered by a local partnership
to cover camera operating costs. Local partnerships are formed by the Police
and Local Authority and others.

The rules governing the operation of the
partnerships are set by the DfT and published in an effectively secret
document.

Amongst the rules are some quite specific
criteria governing the selection of suitable sites for speed camera placement.
Some of the rules can be viewed in the "report of
the two year pilot" (appendix A).

Motorcycle News - 28th January
2004 broke the story...

We gave them an exclusive in return for
large scale coverage.

You can download the MCN article as two
PDFs: (page1) (page2)
new
And there's a brief article on the MCN website (here)

Getting up to speed

The concepts are not particularly easy
to explain, so take your time and read carefully. (If the concepts were
easy, the flaw may have been spotted long ago.) The flaw explains exactly
why everyone thinks that cameras are in the wrong places, yet the DfT and
the camera partnerships are able to insist that speed camera placements
conform with the DfT's own rules. It's one of the rules that's causing
the trouble. Let's leap right in and examine two examples:

Example 1

This is a safe place
to exceed the speed limit. Speed cameras are encouraged in such places.

[this is a 30mph dual carriageway;
part of the A34 at Handforth Dean. Picture courtesy of Peter Edwardson
of speedlimit.org.uk]

Here we examine example numbers that may
apply to this site. See figure 1.

In this example, the 85th percentile speed
is above the prosecution threshold, so speed cameras are permitted. But
many careful and competent drivers are above 35mph and may be prosecuted.
Our best drivers, with the lowest accident risk, may also be prosecuted.
The camera might also catch the occasional nutter who is exceeding a safe
and appropriate speed for the circumstances. But also notice, that since
the 70th percentile speed is at the prosecution threshold, very many safe
drivers driving safely may be prosecuted.

Example 2

This is a dangerous
place to exceed the speed limit. Speed cameras are not allowed.

Here we examine example numbers that may
apply to this site. See figure 2.

In this example, the 85th percentile speed
is below the prosecution threshold, so no speed cameras are permitted.
There might be occasional nutters far in excess of the safe speed for the
circumstances who will avoid prosecution because no cameras are permitted.

The rules for speed camera
placement

Here are some of the rules governing the
placement of speed cameras:

At least 4 KSI per km in last three calendar
years (not per annum)

At least 8 PIA per km in last three calendar
years

Causation factors indicate that speeding was
a contributory factor in some or all of the accidents – sites that are
clearly not speed-related have been de-selected

These curves tell the story of driver competence
and the speeds that drivers choose. It's long been known that the safest
drivers travel generally rather faster than the average speed of traffic.
This is a consequence of their competence and confidence.

Going back to the early 1960s, researchers
observed the U shaped curve of crash risk against speed. This research
has been repeated on numerous occasions, and each time those drivers travelling
somewhat faster than the average were found to have the lowest number of
accident involvements.

Some researchers have failed to find the
U shaped curve, but we know exactly why. There's a discussion on (this
page)

We define points on this curve by reference
to the "percentile". The Oxford English dictionary defines percentile as
follows:

percentile : Each of the 99
intermediate values of a variate which divide a frequency distribution
into 100 groups each containing one per cent of the total population (so
that e.g. 50 per cent have values below the 50th percentile); each of the
100 groups so formed.

On this graph the green trace represents the
number of drivers at each speed. Unsurprisingly, the greatest number of
drivers choose a middle speed (the "50th percentile"). Note the red trace
of crash risk. Research tells us that drivers between the 85th and 90th
percentiles are typically the safest group. We can phrase that in English
as follows: "For any given road situation 90% of drivers do not normally
exceed a safe speed for the circumstances".

We have long used observations of 85th
percentile traffic speeds to set speed limits, and indeed the current DfT
advice on speed limits (In Circular
Roads 1/93) contains important direct references to the 85th percentile
speed.

Traffic engineers around the World agree
that in general an excellent place to set the speed limit is at the 85th
percentile of road traffic speed. The prosecution threshold will largely
protect those safe drivers between the 85th and 90th percentiles. There's
more information on our "speed limits" page,
and a great deal of information around the Internet.

I have
seen welcome promises from you and Mr Jamieson in the media recently about
reviewing the locations of speed cameras. It has been stated that cameras
not placed in accordance with the guidelines will be relocated.

Unfortunately
there is a fatal flaw in the guidelines as I revealed in Motorcycle News
last week.

In
brief, the error is as follows. The rules state that cameras may only be
placed in locations where the 85th percentile traffic speed is above the
ACPO prosecution threshold. Research has long shown that in all normal
circumstances the 85th percentile speed is safe and appropriate for the
road conditions, and that the 85th percentile drivers are the safest.

The
effect of this rule is therefore two fold:

1)
Cameras may only be placed in locations where our safest drivers consider
it reasonable and prudent to exceed the speed limit.

2)
Cameras may not be placed in locations where our safest drivers
consider it dangerous to exceed the speed limit.

This
is exactly the opposite of the desired road safety effect. If cameras were
justified they should clearly be placed in locations where speeding
is dangerous. The flaw means that virtually all speed cameras are
in the wrong places.

This
is a blunder of epic proportions and you must suspend all speed camera
operations immediately pending a complete independent review. The independent
review will find a wide range of serious errors and false assumptions that
underlie the entire principle of speed cameras as road safety devices.

Safe Speed believes that this massive
blunder is a result of muddled and over simplistic thinking behind the
UK's speed cameras.

We believe that those who made the faulty
rule assumed that danger and exceeding the speed limit were equated. Perhaps
they thought: "if more people are exceeding the speed limit it must be
more dangerous".

If we wanted to place cameras in locations
where speeding was dangerous, we should have made the opposite rule: e.g.
You may only place cameras where the 90th percentile of road traffic speed
is below the ACPO prosecution threshold. In this way the
cameras would catch those that were driving faster than the normal speeds
of competent drivers.